function initMap () {
  // 设置token
  Cesium.Ion.defaultAccessToken = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJqdGkiOiIwN2NkNjU1OC01YmJlLTRmM2MtOTJmMy00ZjBiZjU4NmQ2NDkiLCJpZCI6Njk4MDAsImlhdCI6MTYzNTIzNjk3Nn0.GPzJdsSUUmoWPqjv3xZXW678ze9K0E7UQ4yMIK8pFHU';
  // 实例化窗口
  const viewer = new Cesium.Viewer('cesiumContainer', {
    baseLayerPicker: false // 是否显示图层选择器
  })
  // 加载OSM瓦片 https://{s}.basemaps.cartocdn.com/dark_all/{z}/{x}/{y}.png 
  var layer = new Cesium.UrlTemplateImageryProvider({
    url: 'https://{s}.basemaps.cartocdn.com/dark_all/{z}/{x}/{y}.png',
    subdomains: ["a", "b", "c", "d"]
  })
  viewer.imageryLayers.addImageryProvider(layer)
  // 摄像机位移到指定坐标
  // Cartesian3: 3D笛卡尔点 
  // Cartesian3.fromDegrees: 从以度为单位的经度和纬度值返回Cartesian3位置
  viewer.camera.flyTo({
    destination: Cesium.Cartesian3.fromDegrees(116.39084, 39.916413, 2000),
    orientation: {
      heading : Cesium.Math.toRadians(175.0),
      pitch : Cesium.Math.toRadians(-25.0),
      roll : 0.0
    }
  })
  return viewer
}